1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method for fabricating a polysilicon liquid crystal display device. Although the present invention is suitable for a wide scope of applications, it is particularly suited for reducing the number of processes fabricating a liquid crystal display (LCD) device.
2. Description of the Related Art
As interest in image display devices and demand for portable information device increase, a thin film type Flat Panel Display (FPD) devices have been developed that are replacing traditional Cathode Ray Tubes (CRT) type display devices. In particular, a liquid crystal display (LCD) device having the characteristic of optical anisotropy has replaced the CRT. Further, the liquid crystal display device has been used in notebook computers, desktop monitors, or the like because it has an excellent resolution, color rendering capability and picture quality.
A liquid crystal display device includes a color filter substrate, an array substrate and a liquid crystal material formed between the two substrates. A thin film transistor, which uses amorphous silicon or polycrystalline silicon as a channel layer, is used as a switching device of the liquid crystal display device. A process for fabricating the liquid crystal display device usually requires a plurality of photo-mask processes in fabricating the array substrate including the thin film transistor.
FIG. 1 is a plan view showing a unit pixel of an array substrate of a liquid crystal display device of the related art. In a typical liquid crystal display device, the ‘N’ number of gate lines and the ‘M’ number of data lines cross each other, forming the ‘N×M’ number of pixels. For the purpose of simplicity, only one pixel is presented in the FIG. 1. As shown in FIG. 1, the array substrate 10 includes: a pixel electrode 18 formed on a pixel region; gate lines 16 and data lines 17 disposed vertically and horizontally on the substrate 10; and a thin film transistor used as a switching device, which is formed at a region adjacent to where the gates lines 16 and the data lines 17 cross each other.
The thin film transistor includes: a gate electrode 21 connected to the gate line 16; a source electrode 22 connected to the data line 17; and a drain electrode 23 connected to the pixel electrode 18. Also, the thin film transistor includes: a first insulating layer (not shown) and a second insulating layer (not shown) for insulating the gate electrode 21 and source/drain electrodes 22 and 23; and an active layer 24 for forming a conductive channel between the source electrode 22 and the drain electrode 23 by a gate voltage supplied to the gate electrode 21. The source electrode 22 is electrically connected to a source region of the active layer 24 and the drain electrode 23 is electrically connected to a drain region of the active layer 24 through first contact holes 40a formed in the first and second insulating layers. Since a third insulating layer (not shown) has a second contact hole 40b exposing the drain electrode 23, the drain electrode 23 and the pixel electrode 18 are electrically connected to each other through the second contact hole 40b. 
FIGS. 2A to 2G are views showing fabrication processes along the line I-I of the liquid crystal display device shown in FIG. 1 showing fabrication processes according to the related art. As shown in FIG. 2A, the active layer 24 of polysilcon is formed on a transparent substrate 10, such as a glass, by using photolithography.
Next, as shown in FIG. 2B, a first insulating layer 15a is formed over the active layer 24, and a conductive layer 30 is formed on the first insulating layer to form a gate electrode.
Next, as shown in FIG. 2C, a gate electrode 21 is formed on the active layer 24 with the first insulating layer 15a interposed therebetween by patterning the conductive layer 30 using a photolithography process. Thereafter, source/drain regions are formed by injecting N+ or P+ type high density impurity ions into side regions of the active layer 24 using the gate electrode 21 as a mask. The source/drain regions are formed for ohmic contacts to the source/drain electrodes.
Next, as shown in FIG. 2D, after a second insulating layer 15b as an interlayer insulating layer is deposited over the entire surface of the substrate 10 on which the gate electrode 21 is formed, the first contact holes 40a for electrical connection between the source/drain regions and the source/drain electrodes are formed by removing parts of the first insulating layer 15a and the second insulating layer 15b through photolithography.
Thereafter, as shown in FIG. 2E, after a conductive material is deposited over the entire surface of the substrate 10, the source electrode 22 connected to the source region 24S and the drain electrode 23 connected to the drain region 24d are formed through the first contact holes 40a. At this time, a part of the conductive material constructing the source electrode 22 is extended to form the data line.
Next, as shown in FIG. 2F, after the third insulating layer 15c, an organic insulating layer, such as Acryl, is deposited over an entire surface of the substrate 10, a second contact hole 40b is formed to expose the drain electrode 23 by photolithography.
Finally, after transparent conductive materials, such as Indium Tin Oxide (ITO), are deposited over the entire surface of the substrate 10 on which the third insulating layer 15c is formed. The pixel electrode 18 is connected to the drain electrode 23 through the second contact hole 40b an patterned by photolithography.
A process for injecting impurity ions into the active layer to form source and drain regions and a process for activating the impurity ions are required in the related art processes for fabricating the TFT. In order to perform these processes, special equipment is required, which leads to increase fabrication costs in fabricating a thin film transistor. In addition, photolithography processes include a plurality of sub-processes such as photoresist coating, exposure, development and etching etc. As a result, a plurality of photolithography processes lowers production yield and are prone to cause defects in the thin film transistor. Further, since a mask used in the photolithography is very expensive, if many masks are used for the processes, fabrication costs for the liquid crystal display device are also raised.